Direct Solar Thermal Systems with Thermosiphon Frost Protection and Innovative Control Strategies Using a Thermo-Differential Bypass Valve
Abstract
This research involves a direct solar (combi-)system for use in cool climates, based on vacuum-tube collectors, that uses night-circulation for frost protection, which doesn’t just rely on pumped circulation, but also provides frost protection through thermosiphon circulation as a back-up in case of power/control failures. Furthermore, this direct solar system does not use a collector sensor or storage tank sensor, but instead has both the temperature sensors integrated into the pumping station. These features are achieved by using a Thermo-Differential Valve (TDV), which is installed on the storage tank at the solar inlet, and which only opens when the solar supply temperature is higher than the storage tank temperature and bypasses the storage tank when the supply temperature is lower than the storage tank temperature. Novel strategies for operating the circulating pump have been developed and tested, that are designed to optimize the efficiency of the system using the TDV in low-intensity conditions, such as overcast conditions, and to create a system that can focus on generating high temperature heat, without significantly compromising efficiency, by using multiple collector passes to achieve the desired temperature, before directing the flow into the storage tank.